§1.1 Field of the Invention
The present invention concerns data communications. In particular, the present invention concerns cooperative relaying of information in a wireless local area network (LAN).
§1.2 Background Information
In the past decades, WiFi has become one of the most popular wireless technologies due to its low cost, simple installation and great capability to support high speed data communications. The IEEE 802.11 (See, e.g. “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications,” ANSI/IEEE Std 802.11, 1999 Edition, 1999, incorporated herein by reference.) standard has established itself as the most prevalent wireless LANs (WLAN) protocol and includes several versions, such as IEEE 802.11a/b/g/n.
While a conventional WiFi system can support a relatively high data transmission speed (e.g., up to 54 Mbps for IEEE.802.11a/g), the aggregated throughput of a WLAN cell may be severely degraded by slow stations located close to the edge of the cell. (See, e.g., M. Heusse, F. Rousseau, G. Berger-Sabbatel, and A. Duda, “Performance Anomaly of 802.11b,” Proc., IEEE INFOCOM, San Francisco, Calif., April 2003, incorporated herein by reference.) In recent years, the concept of cooperative wireless communication has attracted significant research attention in the physical or layer 1(PHY) layer. (See, e.g. A. Sendonaris, E. Erkip, and B. Aazhang, “User Cooperation Diversity—Part I: System Description,” IEEE Transactions on Communications, Vol. 51, No. 11, pp. 1927-1938 (November 2003); and “User Cooperation Diversity—Part II: Implementation Aspects and Performance Analysis,” IEEE Transactions on Communications, Vol. 51, No. 11, pp. 1939-1948 (November 2003), both incorporated herein by reference.) As one of the MAC layer designs to support a cooperative PHY layer in a WLAN, “CoopMAC” enhances the system throughput by using a two hop transmission, where transmission between a source and a destination occurs via an intermediate station, called a “relay station” or simply a “relay”. (See, e.g. P. Liu, Z. Tao, S. Narayanan, T. Korakis, and S. Panwar, “CoopMAC: A Cooperative MAC for Wireless LANs,” IEEE Journal on Sel. Area in Communications, Vol. 25, No. 2, pp. 340-354 (February 2007); and P. Liu, Z. Tao, Z. Lin, E. Erkip, and S. Panwar, “Cooperative Wireless Communications: A Cross-Layer Approach,” IEEE Communications Magazine, Special Issue on MIMO Systems, (August 2006), both incorporated herein by reference.) The performance of “CoopMAC,” albeit superior to direct communication, is still limited as it only selects a single relay.
To improve a single relay system, multiple relays can be employed at the PHY layer to collaboratively transmit the source signal to the destination, thereby improving diversity gain. Distributed spacetime coding (DSTC) across the relay stations achieves a high spatial diversity while maintaining spectral efficiency. A cooperative MAC layer incorporating DSTC is expected to improve performance over CoopMAC. Unfortunately, however, it still has inherent drawbacks that lead to difficulties and inefficiencies at the MAC layer. Such drawbacks may include, for example, (1) the need to recruit and index relay nodes ahead of time and its associated overhead, (2) the need to estimate accurate channel information for all possible relays and its associated overhead, (3) the need for global information at the source to optimize performance, (4) vulnerability to random loss at the first hop, and (5) the loss of potential diversity and performance gain by unselected relays.
A detailed distributed MAC layer protocol that deploys DSTC in a cooperative ad hoc network is described, for example, in the article G. Jakllari, S. V. Krishnamurthy, M. Faloutsos, P. V. Krishnamurthy, and O. Ercetin, “A Framework for Distributed Spatio-Temporal Communications in Mobile Ad hoc Networks,” Proc., IEEE INFOCOM (Barcelona, Spain, April 2006) (incorporated herein by reference). However, in the Jakllari et al protocol, the source station must (1) discover a set of selected relays and (2) assign the antenna array index to each relay for the underlying DSTC by the use of a broadcast message. Further, each chosen relay, upon receiving that message, must respond with a pilot tone to verify its availability as a relay. This process consumes significant signaling overhead which could be very costly in a mobile environment. Further, under the Jakllari et al protocol, whenever any selected relay fails to receive from the source, DSTC cannot be established and the transmission falls back to direct transmission from the source to the destination. Thus the system robustness to the channel fading and mobility effects is limited. Furthermore, under the Jakllari et al protocol, the source station does not allow stations, other than the chosen set of relays, to cooperate even if those stations may successfully decode the source signal. This sacrifices the potential for additional diversity gains.
The above problems can be addressed by randomized distributed space-time coding (R-DSTC) (See, e.g., B. S. Mergen and A. Scaglione, “Randomized space-time coding for distributed cooperative communication,” IEEE Transactions on Signal Processing, pp. 5003-5017 (October 2007), incorporated herein by reference.), which reduces the requirements for coordination among the source station and the relays. R-DSTC provides robust cooperative relaying of the source signal. More specifically, in contrast to a regular DSTC, R-DSTC does not allocate the antenna array index to each relay. This simplifies the protocol design and reduces signaling costs. A generic cooperative MAC layer protocol is presented in the article P. Liu, Y. Liu, T. Korakis, A. Scaglione, E. Erkip, and S. Panwar, “Cooperative MAC for Rate Adaptive randomized Distributed Space-Time Coding,” Proc., IEEE Globecom (November 2008), and described in U.S. Patent Application Publication No. 2010/0014453 (both incorporated herein by reference), which shows that the throughput gain of R-DSTC over conventional single-hop and two-hop single-relay (e.g. CoopMAC) approaches. However, the Liu et al article does not present a detailed MAC layer design (e.g., for on-the-fly recruitment of relay nodes) and does not describe details of error correction coding and channel coding. Furthermore, for simplicity, channel coding and forward error correction are ignored. Further, in the system described in the Liu article, the transmission rates for the first hop and second hop are picked independently, where each hop rate is based on a packet error rate (PER) threshold. Consequently, there is no guaranteed end-to-end PER for the packet received.
Thus, it would be useful to improve R-DSTC, such as by providing an improved MAC layer design. It would be useful if such a MAC layer design could guarantee end-to-end PER. It would be useful if such a MAC layer design allowed the selection of a transmission rate that meets one or more policy goals.